This invention relates to a fluid pressure control device for a wheel brake which has a small number of components and can be assembled easily.
FIG. 3 shows a conventional fluid pressure control device. Numeral 1 in the figure designates a changeover valve having flow rate control function. Numeral 2 designates an electromagnetic discharge valve. The changeover valve 1 comprises a cylindrical sleeve 4 inserted in a bore formed in a housing 3, a spool 5 slidably mounted in the sleeve 4, and a return spring 6 urging the spool 5 in one direction.
The sleeve 4 has an inlet port 4a communicating with a pressure source (a master cylinder and a pump; neither is shown) and first and second outlet ports 4b and 4c communicating with wheel brakes (not shown).
The spool 5 has a fluid passage 5a therein which places an input chamber 7, in which the front end of the spool is disposed, in communication with a pressure reduction chamber 8, in which the rear end of the spool is disposed, by way of an orifice 9. When the spool 5 is in the illustrated position, i.e. when it is at the extreme end of its movement toward the input chamber 7, fluid from the inlet port 4a passes to the first outlet port 4b without going through the orifice 9. As the spool 5 moves toward the pressure reduction chamber 8, fluid communication between the inlet port 4a and the first outlet port 4b is cut off, while the second outlet port 4c, which has been closed, is brought into communication with the pressure reduction chamber 8 through a groove formed in the outer periphery of the spool 5.
The discharge valve 2 has a frame 10 made of a magnetic material and inserted into the bore of the housing 3 so that it engages the rear end of the sleeve 4 with a liquid-tight seal therebetween. The discharge valve 2 has a fixed valve seat 11 provided in the frame 10 which serves as a magnetic circuit, and a valve member in the form of an armature 14 and valve body 15. By activating a solenoid coil 12, the armature 14, which is biased by a spring 13, is pulled by the magnetic force produced by the coil 12, so that the valve body 15 provided on the front end of the armature moves away from the fixed valve seat 11. The pressure reduction chamber 8 is thus brought into communication with a discharge port 16.
In an ordinary braking mode, the fluid pressures in the input chamber 7 and the pressure reduction chamber 8 are equal to each other, so that the spool 5 is maintained in its original position by the force of the spring 6. Thus, brake fluid is freely movable between the pressure source and the wheel brakes through the fluid passage at a high flow rate.
On the other hand, if the discharge valve 2 is opened in response to a pressure reduction signal for antilock control, the pressure in the pressure reduction chamber 8 will drop. Thus, a difference in fluid pressure will be created between the input chamber 7 and the pressure reduction chamber 8 due to the orifice. This differential pressure moves the spool 5 toward the pressure reduction chamber 8 while compressing the spring 6. Thus, the first outlet port 4b is closed, while the wheel brakes are brought into communication with the discharge port 16 by way of the second outlet port 4c and the pressure reduction chamber 8.
When the discharge valve 2 is closed in response to pressure re-increase signals in the antilock control mode, the spool 5 tends to move to the position where the pressures on both ends thereof balance with each other and returns to a position where the inlet 4a opens a little. In this position, the first outlet port 4b is still closed and the second outlet 4c communicates with the pressure reduction chamber 8. Thus, if the fluid pressure at the inlet 4a is higher than the fluid pressure in the wheel brakes, brake fluid will flow with a low flow rate from the inlet 4a toward the wheel brakes through the orifice 9 and the second outlet 4c. When the difference in pressure between the inlet 4a and wheel brakes decreases below a predetermined value, the spool 5 returns to its original position, so that the inlet 4a is again brought into fluid communication with the first outlet 4b. The abovementioned function of the fluid pressure control device is described in detail in U.S. Pat. No. 4,915,459.
In such a conventional fluid pressure control device, the changeover valve 1 and the discharge valve 2 are separate members and are connected together liquid-tightly in the bore of the housing 3. This control device thus requires a large number of parts and complicated assembling, which are major causes of low productivity.
Also, the use of an O-ring 17 to provide a liquid-tight seal between the sleeve 4 and the frame 10 is not desirable for long-term reliability of the liquid-tight seal and maintenance.